Variable impedance circuit

ABSTRACT

A variable impedance circuit is disclosed which comprises a pair of diodes having their cathodes connected together, a first current source coupled to the connection between the diodes, and a second and a third current source which are connected to the anodes of the diodes respectively. The second and third current sources are controlled by means of current mirror circuits so that equal forward currents are caused to flow therefrom to the paired diodes, thus making it possible to set up a desired impedance variation range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a variable impedance circuit device whose impedance can be varied by means of electric control, and more particularly it pertains to such a circuit device which is so designed that the impedance thereof can be varied over a wide range.

2. Description of the Prior Art

In order to have a better understanding of the present invention, reference will first be made to FIG. 1 of the accompanying drawings, which illustrates a conventional variable impedance circuit which comprises diode-connected transistors Q₁ and Q₂. The illustrated circuit arrangement includes an input terminal 1, a terminal 2 to which is applied a reference voltage, a power supply terminal 3, and current source circuits 5 and 6. The transistors Q₁ and Q₂ have the emitters thereof connected together and to a variable current source 4. With such a circuit arrangement, by changing a control voltage applied to the variable current source 4 to cause a variable current to flow through each of the diode-connected transistors Q₁ and Q₂, a variable impedance element is provided between the terminals 1 and 2. Indicated at 7 is a by-pass capacitor. More specifically, in the circuit arrangement shown in FIG. 1, equal bias currents I₀ flow through the current sources 5 and 6 which are connected to the collectors of the transistors Q₁ and Q₂ respectively so that a current 2I₀ flows through the current source 4 connected to the emitters of these transistors. When a signal source is connected to the terminal 1, an input current i₁ flows in the circuit, and an output current i₂ flows out of it. Under such a condition, the relationship i_(l) =i₂ holds true. Instantaneous input voltage V_(R) applied to the terminal 1 is equal to the difference between base-emitter voltages V_(BE1) and V_(BE2) of the transistors Q₁ and Q₂ respectively. Assuming that a current i_(R) is generated on the basis of the instantaneous input voltage V_(R), then the relationship i_(R) =i₁ =i₂ holds true. Thus, the instantaneous input voltage V_(R) is given by the following expression: ##EQU1## where V_(T) =KT/q; I_(S) is saturation current; K is Boltzmann's constant; T is absolute temperature; and q is electron charge. As will be seen from Equation (1), the instantaneous input voltage V_(R) depends on the input current i₁ flowing in the terminal 1, and the factor thereof can be controlled by means of the equal bias currents I₀. In a small signal range, the impedance R between the terminals 1 and 2 can be determined by differentiating Equation (1) in terms of i_(R) and seeking the gradient of a tangential line with i_(R) =0, and given as follows: ##EQU2## From Equation (2), it will be seen that the impedance R can be proportionally controlled on the basis of the bias currents I₀ which are variable d.c. currents.

However, the foregoing prior-art circuit arrangement is disadvantageous in that since equally controlled variable d.c. currents are applied to the diode-connected transistors Q₁ and Q₂ to cause the bias currents to flow therethrough, there is the tendency that if the equilibrium between the variable d.c. voltages is disturbed, then a current is flown in or drawn in through the terminal 1 or 2 so that the d.c. bias for each of the transistors is suddenly changed, thus imparting distortion to the input signal i₁ which is inputted through the terminal 1. Thus, if the aforementioned prior-art variable impedance circuit arrangement is employed as a noise suppressor circuit, gain control circuit or the like for a radio receiver, then pop noise will disadvantageously be caused due to the sudden d.c. bias changes.

Another disadvantage is such that in the case where the variable current source 4 is comprised of a transistor, it is not possible to flow the control current I₀ by the use of a control voltage lower than about 0.6 V, i.e., the base-emitter voltage of the transistor. In other words, in such a case, the impedance of the circuit shown in FIG. 1 cannot be varied with a control voltage in the range of 0 to 0.6 V or less. Obviously, in the case where the bias circuit for the transistor source 4 is comprised of two diodes connected in series with each other, the impedance cannot be varied with a control voltage lower than about 1.2 V. Thus, the prior-art circuit arrangement shown in FIG. 1 has such a drawback that in the case where the power source voltage V_(CC) is less than 1.8 V, a sufficient quantity of the control current I₀ cannot be flown with a control voltage in the range of 0 to 1.2 V so that the impedance of the circuit can only be varied over a narrow range.

SUMMARY OF THE INVENTION

The present invention is directed to providing a variable impedance circuit which is constructed by the use of elements such as bipolar transistors which can conveniently be incorporated in an integrated circuit, instead of by resorting to the use of field effect transistors.

It is an object of the present invention to provide a variable impedance circuit including a pair of diode-connected transistors or diodes, wherein d.c. potential is prevented from being changed, by flowing equal bias currents through the transistors or diodes.

Another object of the present invention is to provide a variable impedance circuit whose impedance can be varied over a wide range.

Still another object of the present invention is to provide a variable impedance circuit including a pair of PN junctions, wherein the impedance between the input and output terminals of the circuit can be varied over a wide range by flowing a small current through the PN junction pair, thereby preventing distortion from being imparted to signals.

Yet another object of the present invention is to provide a variable impedance circuit which is applicable to recently developed audio devices which can be driven with a power source voltage of 1.8 V or less.

The variable impedance circuit according to the present invention is intended to achieve the foregoing objects.

Other objects, features and advantages of the present invention will become apparent from the ensuing description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a conventional variable impedance circuit.

FIGS. 2 to 5 are circuit diagrams showing embodiments of the present invention, respectively.

FIG. 6 is a view illustrating the control voltage V - forward current I characteristics of variable impedance circuit.

FIGS. 7 to 9 are circuit diagrams showing further embodiments of the present invention, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with respect to embodiments thereof which are shown in FIGS. 2 to 9 wherein similar parts are indicated by like reference numerals and symbols.

Referring first to FIG. 2, there is shown the variable impedance circuit according to a first embodiment of the present invention, which includes a pair of diode-connected transistors Q₁ and Q₂. A transistor Q₄ constitutes a current source circuit and has the collector thereof connected to the emitters of transistors Q₁ and Q₂. A transistor Q₉ is connected between a terminal 3 and the collector of the transistor Q₁ and constitutes, together with transistors Q₇ and Q₈, a first current mirror circuit. Connected between the transistor Q₂ and the terminal 3 is a transistor Q₁₀ which constitutes, together with transistors Q₁₁ and Q₁₂, a second current mirror circuit. Transistors Q₃ to Q₆ constitute a third current mirror circuit serving as current source circuit. The third current mirror circuit is coupled to the first and second current mirror circuits, whereby the bias currents of the diode-connected transistors Q₁ and Q₂ are controlled. By flowing variable current I₀ through the collector of the transistor Q.sub. 6, a collector current, which is equal to the variable current I₀, is caused to flow through each of the transistors Q₃ and Q₅. The emitter area of the transistor Q₄ is made twice as large as those of the transistor Q₃, Q₅ and Q₆ ; thus, a variable current of 2I₀ is caused to flow through the transistor Q₄. The collector of the transistor Q₅ is connected to the bias side of the first current mirror circuit, so that variable current I₀ is caused to flow as collector current of the transistor Q₉ provided at the output side of the first current mirror circuit. The second mirror circuit has the bias side thereof connected to the collector electrode of the transistor Q₃ of the third mirror circuit, so that variable current I₀ is caused to flow as collector current of the transistor Q₁₀ provided at the output side of the second current mirror circuit. Thus, the variable currents I₀ which forwardly flow through the diode-connected transistors Q₁ and Q₂ turn out equal to each other. Indicated at 7 is a by-pass capacitor.

Referring to FIG. 3, there is shown the variable impedance circuit according to a second embodiment of the present invention, which is similar to the circuit arrangement shown in FIG. 2, except that the first and second current mirror circuits are formed by current source circuits comprising multi-collector type transistors Q₁₃ and Q₁₄ respectively.

FIG. 4 shows a third embodiment of the present invention, wherein a single current mirror circuit constituted by transistor Q₁₅ to Q₁₉ is provided in place of the first and second current mirror circuits provided in the embodiment shown in FIG. 2.

FIG. 5 shows a fourth embodiment of the present invention, wherein there is provided a current source circuit which is constituted by a current mirror circuit comprisisng a multi-collector type transistor Q₂₁. Transistors Q₄ and Q₂₀ constitute a current mirror circuit, the transistor Q₄ having an emitter area twice as large as that of the transistor Q₂₀.

As will be appreciated from the foregoing description, according to the present invention, by flowing forward current I₀ in each of the diode-connected transistors, with the aid of current mirror circuits, it is possible to cause equal forward currents I₀ to flow through the transistors. The current mirror circuits may selectively take various forms such as shown in FIGS. 2 to 5, depending on the type of circuit arrangement in which they are used.

With the above-mentioned impedance circuit according to the present invention, by changing variable current I₀ in the range of 1 μA to 1 mA, the impedance R between the terminals 1 and 2 is represented as R=2KT/qI₀ as will be seen from Equation (2). KT/q is about 26 mV at 25° C.; thus, when a current of 1 μA is caused to flow as variable current I₀, the impedance R turns out to be about 50 KΩ, and when a current of 1 mA is caused to flow as variable current I₀, the impedance R turns out to be about 50Ω. In this way, the value of the impedance R can freely be set in such a range.

As will also be appreciated, with the variable impedance circuits shown in FIGS. 2 to 5, forward currents I₀ flowing through the diode-connected transistors can freely be controlled on the basis of a control voltage, with the aid of the current mirror circuits. Generally, in the case where the power source voltage V_(CC) is as low as 1.8 V, the control voltage also becomes low, and thus in such a case, the impedance of the variable impedance circuit can only be controlled in a limited range, or in a worse case, the impedance can no longer be controlled.

In order to eliminate such a drawback, according to the present invention, there are also provided variable impedance circuits such as shown in FIGS. 7 to 9, each of which is so designed that the impedance thereof can be controlled even with a low power source voltage.

Each of the variable impedance circuits shown in FIGS. 7 to 9 includes two current source circuit systems, wherein variable currents derived from the two current source systems are superimposed upon each other and then supplied to a current mirror circuit which is arranged to control the bias current of the variable impedance circuit, thereby making variable the impedance of the circuit.

The variable impedance circuit shown at 10 in FIG. 7 comprises diodes D₁, D₂, current sources 8, 9, a current source transistor Q₄, and an emitter resistor R₅. Indicated at 11 is an input terminal to which an audio signal is inputted. A rectifier circuit 12 is provided which comprises a diode and smoothing capacitor and which may take the form of a full-wave rectifier. A buffer amplifier 13 is provided, and divider resistors R₁ and R₂ are connected in series between the output terminal P₁ of the buffer amplifier 13 and ground. The connection point P₂ between the resistors R₁ and R₂ is coupled to the base of a transistor Q₂₂ ; a resistor R₄ is connected to the emitter of the transistor Q₂₂ ; and the collector of the transistor Q₂₂ is tied to the collector of a transistor Q₂₃. A current mirror circuit 14 is constituted by the transistor Q₂₃ and a transistor Q₂₄, and a resistor R₃ is connected between the collector of the transistor Q₂₄ and the connection point P₁. A bias circuit 15 which comprises diode-connected transistors Q₃₀, Q₃₁ is connected between ground and the connection point P₃ between the transistor Q₂₄ and the resistor R₃. The connection point P₃ is coupled to the base of the current source transistor Q₄.

A signal voltage e₀ such as audio signal or the like, which is applied to a terminal 11, is rectified and smoothed by the rectifier circuit 12, and then passed through the buffer amplifier 13 to be converted to a variable d.c. control voltage e₁ which in turn is applied to the first and second current source circuits. The first current source circuit will first be explained. The variable d.c. control voltage e₁ derived from the buffer amplifier 13 is applied across the divider resistors, and when a divided voltage e₂ obtained at the connection point P₂ between the divider resistors exceeds the threshold level of the transistor Q₂₂, the latter is rendered conductive so that variable current I₁ is caused to flow as collector current of the transistor Q₂₂. The variable current I₁ is passed to the bias circuit 15 through the current mirror circuit 14. The transistor Q₄ is biased by the bias circuit 15 so that a current proportional to the variable current I₁ is caused to flow therethrough. When the variable d.c. control voltage e₁ exceeds the threshold level of the bias circuit 15, variable current I₂ is caused to flow in the bias circuit 15 through the resistor R₃ so that a current proportional to the variable current I₂ is caused to flow as collector current of the transistor Q₄. In this way, a current 2I₀ proportional to the currents I₁ and I₂ is caused to flow in the variable impedance circuit. The threshold voltage of the bias circuit 15 is about 1.2 V since the bias circuit 15 is constituted by the series connection of the diode-connected transistors Q₃₀, Q₃₁.

As mentioned above, the variable impedance circuit of FIG. 7 includes the first circuit for controlling, in a low current range, the bias currents or forward currents flowing through the diodes D₁ and D₂ ; and the second circuit for generating a current when the signal voltage e₁ exceeds the threshold voltage of the bias current 15, thereby controlling, in a high current range, the bias currents mentioned above.

Reference will now be made to FIG. 6, wherein a straight line a indicates the voltage-current characteristic of the first current source circuit; a straight line b indicates the voltage-current characteristic of the second current source circuit; and a bent line c shows the characteristic of variable current 2I₀ flowing through the current source transistor Q₄ of the variable impedance circuit 10, the variable current 2I₀ being an output current obtained on the bias of bias current resulting from superimposition of the variable currents I₁ and I₂ derived from the first and second current source circuits respectively.

Description will now be made of the operation of the variable impedance circuit shown in FIG. 7. A sinusoidal signal e₀ such as audio signal or the like is applied to the input terminal 11 and rectified in the rectifier circuit 12 so that a voltage e₁, which is substantially smoothed to be d.c., is obtained through the buffer amplifier 13. The voltage e₁ thus obtained is applied to be divided by the divider resistors R₁ and R₂ ; thus a divided voltage e₂ is obtained. When the divided voltage e₂ becomes higher than the base-emitter voltage V_(BE1) of the transistor Q₂₂, the latter is turned on so that variable current I₁ is caused to flow as collector current of the transistor Q₂₂. Further, variable current I₁ is caused to flow through the transistor Q₂₃ of the current mirror circuit 14, and at the same time variable current I₁ is caused to flow as collector current of the transistor Q₂₄, which the bias circuit 15. At this point, the current source transistor Q₄ is biased by bias voltage generated in the bias circuit 15, so that variable current 2I₀ proportional to the variable current I₁ is caused to flow as collector current of the transistor Q₄. The variable current 2I₀ is given as follows: ##EQU3## where V_(F) is the sum of barrier voltages of the diode-connected transistors Q₃₀ and Q₃₁, and V_(BE) is the base-emitter voltage of the transistor Q₄.

When the signal voltage e₁ builds up and exceeds the threshold votage (about 1.2 V) of the bias circuit 15, the variable current I₁ is added to the variable current I₁ and flows in the bias circuit 15. The transistor Q₄ is biased by bias voltage generated in the bias circuit 15, and thus a current in proportion to the added variable current (I₁ +I₂) is caused to flow as collector current of the transistor Q₄. The variable currents I₁ and I₂ are given by the following equations: ##EQU4## where V_(BE1) is the base-emitter voltage of the transistor Q₂₂.

Referring to FIG. 8, there is shown the variable impedance circuit according to a sixth embodiment of the present invention, which is similar to the circuit of FIG. 2 except that a first and a second circuit for obtaining control currents are additionally provided. Transistors Q₃, Q₄ and Q₅ constitute, together with a bias circuit 15, a constant current source circuit. Current mirror circuits 19 and 20 are supplied with control currents I₀ available from the constant current source circuit, so that equal forward currents I₀ are caused to flow through diode-connected transistors Q₁ and Q₂ respectively. Control current (I₁ +I₂) which flows in the bias circuit 15 is similar to that in the embodiment shown in FIG. 7.

Referring to FIG. 9, there is shown the variable impedance circuit according to a seventh embodiment of the present invention, which includes a current mirror circuit 16 comprising transistors Q₂₇ to Q₂₉ ; a current mirror circuit 17 comprising transistors Q₁₅ to Q₁₉ ; and a current mirror circuit 18 comprising transistors Q₄, Q₂₅ and Q₂₆.

A signal voltage e₀ supplied via an input terminal 11 is rectified in a rectifier circuit 12, and thus a control voltage e₁ is obtained through a buffer circuit 13. When the level of a bias voltage obtained by means of divider resistors R₁ and R₂ is not high enough to turn on a transistor Q₂₂, a current I₂ (equal to e₁ /R₃) flowing through a resistor R₃ flows in the current mirror circuit 16. The current I₂ is also caused to flow as forward current of diode-connected transistors Q₁ and Q₂ through the current mirror circuit 17 and 18. A current 2I₀ is caused to flow through the current source transistor Q₄. When the control voltage e₁ builds up so that the transistor Q₂₂ is turned on, this transistor Q₂₂ draws in, as collector current, the current I₁ from the current mirror circuit 17. Consequently, a control current (I₁ +I₂) is caused to flow through the current mirror circuit 17, whereby the forward current I₀ is controlled. In this way, such a V-I characteristic as shown by the curve c in FIG. 6, is achieved. Thus, it is possible to make variable the impedance of the variable impedance circuit even when the power source voltage is as low as 1.8 V or less. By using such a concept, it is possible to achieve any desired V-I characteristic so that the extent of impedance variation can freely be set.

As will be appreciated from the foregoing discussion, with the variable impedance circuit according to the present invention, forward currents that flow through the impedance elements formed by a pair of diode-connected transistors or diodes are maintained to be equal to each other by the use of current mirror circuits, so that the impedance between the terminals 1 and 2 can be varied. By controlling the forward currents in this way, the d.c. level when the impedance is varied, is stabilized so that there occurs no distortion due to potential difference between the terminals 1 and 2 of the variable impedance circuit. Thus, by employing the variable impedance circuit of the present invention with an audio signal processing system for a radio receiver or the like, for example, the problem of pop noise occurrence can most effectively be avoided.

The variable impedance circuit according to the present invention is also advantageous in that the same can be comprised of elements that are well adapted to be fabricated in the form of a semiconductor integrated circuit.

A further advantage is that the present variable impedance circuit can readily be programmed in terms of its impedance varying range by virtue of the fact that forward currents that flow through the impedance elements formed by transistors or diodes are supplied through current mirror circuits and thus can easily be set.

A still further advantage is that the impedance can be varied even with a power source voltage as low as about 1.8 V.

While the present invention has been described and illustrated with respect to some specific embodiments thereof, it is to be understood that the present invention is by no means limited thereto but covers all changes and modifications which will become possible within the scope of the appended claims. 

What is claimed is:
 1. A variable impedance circuit, comprising:a pair of diodes connected with each other; a first current source circuit coupled to the connection point between said diodes; a second and a third current source circuit connected to the anodes of said diodes respectively; terminals provided at the anode sides of said diodes respectively so that a variable impedance element is established therebetween; a first current mirror circuit for equally controlling forward currents flowing from said second and third current source circuits to said diodes respectively; a second current mirror circuit for flowing, through said first current source circuit, a current substantially equal to the sum of the forward currents flowing out of said second and third current source circuits; a transistor driven with a control voltage higher than a predetermined level to draw in a current from said first current mirror circuit; and a third current mirror circuit driven with a control voltage lower than a predetermined level to draw in a current from said first current mirror circuit; wherein currents derived from said transistor and said third current mirror circuit are caused to flow, through said first current mirror circuit, in said pair of diodes as equal forward currents, thereby making variable the impedance between said terminals.
 2. A variable impedance circuit according to claim 1, wherein said pair of diodes are constituted by diode-connected transistors. 